1. Field of the Invention
The present invention relates to a semiconductor lighting apparatus using a plurality of semiconductor light emitting devices, particularly to a technique for maintaining a luminance and a chromaticity to be constant.
2. Description of the Background Art
A semiconductor light emitting device such as a semiconductor laser (a laser diode: LD) or a light emitting diode (LED) is used in place of a lamp as a light source of an illuminating device or a display device. For uses of this type, there is utilized a light emitting device corresponding to three colors of red, green, and blue colors which are so-called three primary colors of light. By controlling a rate of quantities of the light having the respective colors to be a constant ratio and adding them together, white light is obtained by synthesis. In order to implement excellent color reproducibility, it is necessary to maintain the rate of the quantities of the light having the respective colors to be constant and to hold respective wavelengths of the light having the respective colors to be constant.
The semiconductor light emitting device has such a property that an emission wavelength is changed due to a variation in a junction temperature which is caused by a change in a driving current or an environmental temperature. For this reason, there is a problem in that a chromaticity is changed due to the variation in the junction temperature even if the driving current is set to be constant so as to hold the rate of the light quantities to be constant.
Against the problem, International Publication WO2011/086682 discloses a technique for adjusting a cooling capacity of each light source to limit a rate of light quantities of respective light sources into a certain range even if a junction temperature is varied, thereby maintaining white balance. If a driving current is changed to regulate the light quantity, an emission wavelength is also varied. For this reason, the driving current is fixed to control the cooling capacity.
Moreover, Japanese Patent No. 4525767 proposes that a junction temperature is calculated from an ambient temperature of an LED and a target light quantity with which the white balance is maintained is calculated from a difference between the junction temperature and a reference temperature.
Both the International Publication WO2011/086682 and the Japanese Patent No. 4525767 have an object to maintain the white balance (the rate of the quantities of the light having the respective colors) and do not prevent the change in the emission wavelength of each light emitting device which is caused by the variation in the temperature. Further, in the International Publication WO2011/086682, the driving current is fixed. For this reason, even if the rate of the light quantities can be maintained, it is impossible to regulate a light output value to have an optional constant value.